In a wireless communication network, when a frequency band is shared for transmission by multiple users, interference may arise on the signal of one user due to the presence of the signals of the other users. In this case, the quality of the signal of a user may be strongly degraded. An example of a wireless network potentially affected by the problem above is a UMTS (Universal Mobile Telecommunications System) network employing the W-CDMA (Wideband Code Division Multiple Access) technology. In particular, in a W-CDMA access network, a base station receives signals from a number of near and far mobile stations. If all the mobile stations transmit at the same power level, signals from near mobile stations will be received by the base station at high power level, while signals from remote mobile stations will have a much lower power level. Communication from far mobile stations will be strongly impaired by the interference generated by near mobile stations. This effect is commonly known as the “near-far problem”.
Typically, in a UMTS network, a technique known as transmission power control is applied. This technique allows controlling the transmission power of the mobile stations on the uplink such that the received power levels at the base station from near and far mobile stations allow to achieve target values of the signal to interference and noise ratio (SINR) for the communication links established between the mobile stations and the base station, regardless of the position of the mobile stations.
Moreover, as known, received power at a base station fluctuates rapidly due to small scale fading effects in the communication links. Therefore, a closed loop transmission power control system is typically used, which employs transmission power control bits to set the transmitted power at a mobile station on the basis of the received SINR. In particular, when a mobile station in a cell communicates with the corresponding base station, the base station estimates the received SINR of the communication link, compares the received SINR with the target SINR set for the communication link, and determines the transmission power control bits accordingly. Subsequently, the base station sends the transmission power control bits through a feedback channel to the mobile station. Upon reception of the feedback signal from the base station, the mobile station extracts the transmission power control bits and determines the transmission power. The closed loop thus formed between the base station and the mobile station enables to adjust the transmission power in order to keep the received SINR constant at the target level even in presence of the small scale fading effects mentioned above.
Several techniques for estimating the SINR at a base station are known.
WO 02/087106 A1 discloses a method and a device for estimating the signal to interference ratio of a signal, in particular in a base station of a WCDMA system, providing at least a first estimating process and a second estimating process, wherein for current use with the signal a selection among said estimating processes is made in accordance with a current characteristic or current characteristics of said signal so that an estimating process is selected which is most suitable in view of the current characteristic(s) of said signal.
According to WO 02/087106 A1, a known method for estimating the SIR (signal to interference ratio) may be described as follows. According to this method, the SIR is estimated by using a narrowband interference estimation. The signal to interference ratio is described by:
                    SIR        =                  S          I                                    [        1        ]            where S is the received signal power of a dedicated physical control channel (DPCCH) and I is the interference power including a system thermal noise. Equation [1] requires the estimation of the signal power and the interference power. The signal power can be estimated using the pilot symbols in the DPCCH channel separately for each receiving antenna as:
                              S          ′                =                                            ∑                              k                =                1                            L                        ⁢                          S              k              ′                                =                                    ∑                              k                =                1                            L                        ⁢                                                                                                1                                          N                      p                                                        ⁢                                                            ∑                                              i                        =                        1                                                                    N                        p                                                              ⁢                                          z                                              k                        ,                        i                                                                                                                        2                                                          [        2        ]            where L is the number of allocated fingers for current antenna (number of received paths), Np is the number of pilot symbols, and z is the sample (complex value) of the DPCCH channel, from which the pilot modulation is removed.
The signal power estimate is biased by the second term in the following equation:
                              E          ⁡                      (                          S              ′                        )                          =                  S          +                                                    I                ·                L                                            N                p                                      .                                              [        3        ]            
The bias can be removed after the antennawise noise and interference power I is estimated, with the following equation:
                              S          ub          ′                =                              S            ′                    -                                                    I                ·                L                                            N                p                                      .                                              [        4        ]            
The power of the noise and interference term I is determined considering that, under the assumption that the power of the transmitted narrowband signal and the channel power remain constant over the calculation period (i.e. a timeslot), the variance of the received signal is equal to the variance of the noise and interference. The interference power estimate is given by:
                              I          ′                =                              1            L                    ⁢                                    ∑                              k                =                1                            L                        ⁢                          I              k              ′                                                          [        5        ]            where:
                              I          k          ′                =                                            1                              N                p                                      ⁢                                          ∑                                  i                  =                  1                                                  N                  p                                            ⁢                                                z                                      k                    ,                    i                                                  ·                                  z                                      k                    ,                    i                                    *                                                              -                                                                                    1                                      N                    p                                                  ⁢                                                      ∑                                          i                      ⁢                                                                                          ⁢                      01                                                              N                      p                                                        ⁢                                      z                                          k                      ,                      i                                                                                                          2                                              [        6        ]            where zk,i* is the complex conjugate of the despread and demodulated pilot symbol zk,i.
To reduce the variance of the interference and noise power estimate of a current antenna, the estimate can be filtered using a 1-tap IIR filter, resulting in the following equation:I′fil(t)=k·I′(t)+(1−k)·I′(t−1)  [7]
Where index t refers to a current timeslot and (t−1) to a previous timeslot. The Kalman gain k of the filter should be 0.25.
Using equations [4] and [7] in equation [1], the SIR of one antenna may be estimated as:
                              SIR          ′                =                                            S              ub              ′                                      I              fil              ′                                =                                                                      S                  ′                                -                                                                            I                      fil                      ′                                        ·                    L                                                        N                    p                                                                              I                fil                ′                                      .                                              [        8        ]            
U.S. Pat. No. 6,404,826 B1 discloses a circuit designed with an estimate circuit coupled to receive a plurality of predetermined signals from an external source. Each of the predetermined signals is spaced apart in time. The estimate circuit produces a first estimate signal in response to at least one of the plurality of predetermined signals. An averaging circuit is coupled to receive a data signal and at least one of the plurality of predetermined signals. The averaging circuit produces an average signal from the data signal and at least one of the plurality of predetermined signals.
WO 99/46869 discloses using measured estimates of received signal energy and interference values to determine a measured signal-to-interference ratio (SIR) value. The measured SIR value is corrected for non-linearity to obtain a corrected SIR value using a correction function. That corrected SIR value may then be used in any number of applications such as to control the transmit power of mobiles in a mobile radio communications system.
WO 2004/051902 discloses a method for providing an unbiased signal-to-interference (SIR) estimation in a radio communication system. The method comprises the steps of receiving a transmitted signal, estimating the signal energy and an interference value of the received signal, calculating a first SIR estimate based on the estimated signal energy and the interference value, correcting the first estimated SIR-value for nonlinearity by means of a correction function and thereby obtaining a corrected SIR-value, while taking inter-path interference into account, repeatedly calculating the corrected SIR-value on the basis of received signal samples, whereby the same accuracy is achieved regardless of the number of paths occurring in the receiver.